Determining Precipitation Type

1
DeterminingPrecipitation Type
2
Determining Precipitation Type

• Rain (R, RA)- Rain is liquid precipitation that
  reaches the surface in the form of drops that are
  greater than 0.5 millimeters in diameter.
• The intensity of rain is determined by the
  accumulation over a given time.
• Categories of rain are light, moderate and heavy.

3
Cold Rain Sounding
4
Cold Rain Sounding
5
Determining Precipitation Type

• Snow (SN, SNW, S)- Snow is an aggregate of ice
  crystals that form into flakes.
• Snow forms at temperatures below freezing. For
  snow to reach the earth's surface the entire
  temperature profile in the troposphere needs to
  be at or below freezing.
• It can be slightly above freezing in some layers
  if the layer is not warm or deep enough the melt
  the snow flakes much.
• The intensity of snow is determined by the
  accumulation over a given time. Categories of
  snow are light, moderate and heavy.

6
Snow Sounding
7
Snow Sounding
8
Determining Precipitation Type

• Snow Pellets (GS)- A snow pellet is precipitation
  that grows by supercooled water accreting on ice
  crystals or snow flakes.
• Snow pellets can also occur when a snowflake
  melts about half way then refreezes as it falls.
  Snow pellets have characteristics of hail, sleet
  and snow.
• Snow pellets will crush and break apart when
  pressed.
• They can bounce off objects like sleet does. Snow
  pellets have a whiter appearance than sleet.
• Snow pellets have small air pockets embedded
  within their structure and have visual remnants
  of ice crystals unlike sleet. Snow pellets are
  typically a couple to several millimeters in
  size.

9
Determining Precipitation Type

• Snow grains are small grains of ice. They do not
  produce much accumulation and are the solid
  equivalent to drizzle.
• Ice Crystals (IC)- Also called diamond dust. They
  are small ice crystals that float with the wind.

10
Determining Precipitation Type

• Sleet / Ice Pellets (PE, PL, IP, SLT)- Sleet (Ice
  Pellets) are frozen raindrops that strike the
  earth's surface.
• In a sleet situation the precipitation aloft when
  it is first generated will be snow.
• The snow falls through a layer that is a little
  above freezing and the snow partially melts.
• If the snow completely melts it will be more
  likely to reach the earth's surface as
  supercooled water instead of sleet.
• If the snow partially melts there will still be
  ice within the falling drop for water to freeze
  on when the drop falls into a subfreezing layer.
  The lowest layer of the troposphere will be below
  freezing in a sleet situation and deep enough to
  freeze drops completely.

11
Sleet Sounding
12
Sleet Sounding
13
Determining Precipitation Type

• Hail (GR, A)- Hail is dense precipitation ice
  that is that least 5 millimeters in diameter.
• It forms due to ice crystals and supercooled
  water that freeze or stick to the embryo hail
  stone.
• Soft hail is more white and less dense since it
  has air bubbles. Soft hail occurs when hail grows
  at a temperature below freezing by ice crystals
  and small supercooled water and cloud droplets
  merging onto the hail.
• Hard hail occurs when liquid water drops freeze
  on the outer edges of the hailstone after the
  outer edge is above freezing.
• The freezing of supercooled water releases latent
  heat and this can result in the outer edge of the
  hail stone warming above freezing. Then the water
  refreezes creating solid ice. Hail will commonly
  have soft ice and hard ice layers when it is
  sliced open.

14
Determining Precipitation Type

• Graupel (GS)- Graupel forms in the same way as
  hail except the diameter is less than 5
  millimeters. It usually grows by soft hail
  processes.
• Drizzle (DZ, L)- Drizzle is liquid precipitation
  that reaches the surface in the form of drops
  that are less than 0.5 millimeters in diameter.

15
Determining Precipitation Type

• Freezing Drizzle (FZDZ, ZL)- Freezing Drizzle is
  liquid precipitation that reaches the surface in
  the form of drops that are less than 0.5
  millimeters in diameter. The drops then freeze on
  the earth's surface.
• Freezing Rain (FZRA, ZR)- Freezing Rain is liquid
  precipitation that reaches the surface in the
  form of drops that are greater than 0.5
  millimeters in diameter. The drops then freeze on
  the earth's surface.

16
Freezing Drizzle Sounding
17
Freezing Drizzle / Freezing Rain Sounding
18
Freezing Rain Sounding
19
Freezing Rain Sounding
20
Determining Precipitation Type

• Freezing Fog (FZFG)- Freezing fog is a fog
  composed of supercooled water drops. These drops
  freeze just after they wet the earth's surface.
• Mixed Precipitation (MXD PCPN)- The combination
  of two or more winter precipitation types
  occurring at the same time or over a period of
  time at the same place.

21
Ice Crystal Formation

• Three processes can cause ice crystal formation
  in a cloud
• Heterogeneous nucleation
• Deposition
• Ice Multiplication

22
Ice Crystal Formation

• Heterogeneous nucleation is the process by which
  ice crystals form from liquid water molecules as
  the molecules collect and freeze onto foreign
  particles, such as dust, clay, and aerosols
• Heterogeneous nucleation is most likely at
  temperatures less than -10 C (especially around
  -15 C), but can occur as warm as -5 C
  heterogeneous nucleation accelerates the freezing
  of drops, i.e., if no foreign particles are
  present, the air must be much colder for freezing
  to occur

23
Ice Crystal Formation

• Heterogeneous nucleation cont.
• However, ice embryos also can pre-exist in
  clouds, which can freeze drops at temperatures
  warmer (around -5 C) than those associated with
  heterogeneous nucleation freezing.
• If freezing occurs by this process, larger drop
  diameters tend to have higher/warmer freezing
  temperatures (e.g., a 1.0 mm drop freezes at
  about -12 C) than smaller drops (e.g., a 0.2 mm
  drop freezes at about -20 C), since large drops
  have more freezing nuclei/particles in them

24
Ice Crystal Formation

• Deposition is the process by which ice crystals
  form directly from the water vapor stage in a
  saturated, subfreezing cloud, although deposition
  nuclei (centers upon which ice crystals form)
  must be present
• The number of crystals forming in a cloud via
  deposition is a function of temperature. At
  temperatures greater than -10 C, not enough
  deposition nuclei are present to form enough
  crystals for an efficient precipitation process
  to occur, although crystals can still form.
• Temperatures colder than -10 C, i.e., -10 to -20
  C, are necessary for efficient crystal formation
  via the deposition process

25
Ice Crystal Formation

• Deposition Cont.
• The deposition process is maximized at
  temperatures of -12 to -15 C when coincident with
  saturated ascent. At these temperatures, ice
  crystals grow at the expense of water droplets,
  as water vapor molecules migrate to the crystals.
  
• Also, these temperatures typically are found in
  the 700 to 500 mb layer in winter, i.e., where
  ascent usually is maximized near the level of
  non-divergence in winter storms.
• Deposition can produce cloud size crystals around
  1 mm but this generally is not large enough to
  produce much surface precipitation

26
Ice Crystal Formation

• Ice multiplication results in many more crystals
  in a cloud than that caused by heterogeneous
  nucleation or deposition alone.
• Ice multiplication results in fragmented or
  splintered ice crystals ice (rime) splintering
  occurs at temperatures warmer than -10 C while
  mechanical fracture of ice occurs at colder than
  -10 C

27
Ice Crystal Formation

• Ice (rime) splintering is the primary ice
  multiplication mechanism and is common at
  temperatures from 0 to -10 C with a peak at about
  -5 C.
• Splintering occurs when crystals originating in
  saturated layers aloft that are colder than -10 C
  (from either heterogeneous nucleation or
  deposition) fall into a 0 to -10 C layer where
  many supercooled droplets exist.
• The droplets freeze as they contact the crystals,
  then tend to break up (fragment/splinter) if
  large enough as the crystal/frozen droplet
  particles travel through the cloud

28
Ice Crystal Formation

• Ice multiplication via splintering is common
  where low and middle-level saturated ascent is
  occurring with mean temperatures about -5 C
  (e.g., in a comma head of a mature cyclone or
  where strong isentropic ascent and frontal
  forcing are occurring).
• In these areas, a lot of crystals and nuclei are
  present due to ice multiplication which, due to
  strong ascent, rise into layers with temperatures
  less than -10 C.
• This in turn leads to additional crystal
  formation and deposition which can then result in
  even more ice multiplication and so on. The
  result is that bursts of heavy snow can occur

29
Ice Crystal Growth

• Ice Crystal Types
• Ice crystals come in different types and shapes
  which are dependent on the temperatures in which
  they form and grow. The shapes of crystals and
  their associated temperature range include

30
Ice Crystal Growth

• Dendritic crystals are the dominant type in many
  snow events as they form in a range of
  temperatures that is most conducive to deposition
  and maximum ascent in the 700 to 500 mb layer
• Ice crystal formation and growth by deposition
  alone typically cannot produce much precipitation
  sized particles. Ice crystal growth to
  precipitation sized particles (about 10 mm) is
  achieved via riming and aggregation. Growth rate
  increases as the size of ice particles increases

31
Ice Crystal Growth

• Riming
• Growth by riming occurs and is most efficient
  when crystals fall from a cold layer aloft (less
  than -10 C) and contact supercooled water
  droplets in a saturated 0 to -10 C layer below,
  causing freezing of the droplets. Many of these
  particles then break apart via ice multiplication
  and splintering resulting in many different sized
  crystals. Excessive riming can produce snow
  graupel and sleet

32
Ice Crystal Growth

• Aggregation
• Aggregation is a very important method for
  crystal growth, and occurs when the terminal fall
  speeds of varying crystal shapes are different
  resulting in crystal collisions and subsequent
  growth
• Fall speeds generally are strongly dependent on
  the shapes and sizes of crystals, which is a
  function of the temperatures in which they are
  growing (which affects shape) and the amount of
  riming (which affects size) crystals have
  undergone. For dendrites, fall speed is
  independent of crystal diameter for columns and
  graupel, fall speed is dependent on diameter

33
Acknowledgements

• Portions of this lecture were adapted from
  material found at the following locations
• Meteorology Today Textbook
• Severe and Hazardous Weather Textbook
• Atmospheric Science, An Intro Survey Textbook
• NWS Louisville Science and Technology web site
• http//www.crh.noaa.gov/lmk/soo/docu/precip_type.h
  tm
• http//www.crh.noaa.gov/lmk/soo/
• The Weather Prediction web site
• http//www.theweatherprediction.com
• http//theweatherprediction.com/preciptypes/